Isotopes are atoms of the same element but different atomic masses. They have different atomic masses because they have different numbers of neutrons.
For oxygen, you can find atoms of oxygen with a mass of 16 and 18 but they all have the atomic number of 8. This means there is an oxygen atom that has eight neutrons in its nucleus and one with ten.
Isotopes are usually not as common and are quite often radioactive.
This topic is also very important for studying radiation in Physics so well worth getting to grips with.
You never need to remember the periodic table, it will be given to you in the exam.